Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as properties, low specific gravity, high transparency and low yellowness, and as optical properties, high refractive index and high Abbe number, and high heat resistance, high strength, etc. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. High strength facilitates secondary processing and is important in terms of safety, etc. Examples of the technique of simultaneously achieving high refractive index and high Abbe number as optical properties and high heat resistance include use of an episulfide compound, but optical materials made of the compound have a problem in that it is difficult to obtain high strength. For this reason, the improvement of strengths such as tensile strength and impact resistance by the addition of a thiol compound and an isocyanate compound to an episulfide compound has been examined (see Patent Documents 1-4).
[Problem 1]
In the case of the polymerization reaction of the composition in which a thiol compound and an isocyanate compound are added to an episulfide compound, various reactions such as a simple polymerization reaction of an episulfide group, a reaction between an episulfide group and a thiol group, a reaction between an episulfide group and an isocyanate group and a thiourethanation reaction between a thiol group and an isocyanate group proceed simultaneously or successively. For this reason, there are problems in that, for example, a portion with non-uniform refractive index may be generated in a resin, resulting in distortion or white turbidity in a cured product. Further, these optical materials are usually produced by injecting a polymerizable composition in which a polymerization catalyst is added into a forming die, which is produced using a mold made of metal or glass and a resin gasket or adhesive tape, and then thermally polymerizing the composition. However, resin gaskets are expensive. For this reason, recently, methods using an inexpensive adhesive tape instead of a resin gasket have been mainly employed industrially (see Patent Document 5). However, when heating and curing with a mold in which the peripheral portions of 2 molds are wrapped with an adhesive tape, depending on the type of an adhesive component, there may be a problem in that the adhesive component is eluted in the composition, resulting in white turbidity of the obtained optical material. For this reason, methods for producing an optical material, in which a thiol compound and an isocyanate compound are prepolymerized in the presence or absence of an episulfide compound and then the remaining episulfide compound is added thereto to be polymerized, have been proposed (see Patent Document 6). However, when using tetrabutylphosphonium bromide exemplified in the Examples of Patent Document 6 as a prepolymerization catalyst, not only the reaction between the thiol compound and the isocyanate compound, but also the polymerization reaction of the episulfide compound proceed simultaneously, and the viscosity of the composition is increased to 3000 cP or more, causing the problem in that it is extremely difficult to perform cast molding. Therefore, it is inappropriate for industrially producing optical materials.
[Problem 2]
Moreover, the optical material made of the episulfide compound tends to be easily yellowed at the time of the heating treatment for the surface treatment of the optical material, after long-term storage and after long-term use. In particular, a method for suppressing yellowing under conditions for the surface treatment of the optical material requiring heating at 100° C. or higher for 10 hours or longer and under conditions for the acceleration test for long-term storage has been desired. For this reason, as methods for suppressing yellowing of the optical material made of the episulfide compound, a technique for the addition of a thiol compound (Patent Document 7), a method for the addition of a compound having a NH2 group and/or a compound having a NH group (Patent Document 8) and methods for the addition of phenols (Patent Documents 9 and 10) have been proposed. However, the composition in which the thiol compound and the isocyanate compound are added to the episulfide compound often causes problems in that, for example, the effect of suppressing yellowing may be insufficient, heat resistance may be significantly reduced, and because the polymerization reaction of the episulfide compound and the thiourethanation reaction proceed rapidly and randomly, a portion with non-uniform refractive index may be generated in a resin, resulting in distortion or white turbidity in a cured product. Meanwhile, as a method for suppressing yellowing of the optical material made of a composition in which the thiol compound is added to the episulfide compound, a method for the addition of a 2,2,6,6-tetramethylpiperidine compound not having a polymerizable group (Patent Document 11) has been proposed. However, though in the composition in which the thiol compound and the isocyanate compound are added to the episulfide compound, the effect of suppressing yellowing is exerted thereby, the compatibility with the optical material made of the composition in which the thiol compound and the isocyanate compound are added to the episulfide compound is bad, and there are problems in that, for example, white turbidity is generated in the optical material, and the 2,2,6,6-tetramethylpiperidine compound not having a polymerizable group is bled out to the surface of the optical material, which significantly worsens the outer appearance. As a method for improving weather resistance of the composition in which the thiol compound and the isocyanate compound are added to the episulfide compound, a method for the addition of p-toluenesulfonamide (Patent Document 12) has been proposed. However, the effect of suppressing yellowing under heating conditions is insufficient, and there are problems in that, for example, white turbidity may be generated in the optical material.
In consideration of the above-described matters, the development of a catalyst for preliminary reaction and preliminary reaction conditions, which enable preferential progress of only a thiourethanation reaction of a composition in which a thiol compound and an isocyanate compound are added to an episulfide compound almost without progress of a polymerization reaction of the episulfide, was desired. In addition, the development of a technique of suppressing yellowing of an optical material made of a composition in which a thiol compound and an isocyanate compound are added to an episulfide compound was also desired.